It's actually possible to do this on a jailbroken device.
I had a discussion about this and the consensus was you're probably on better legal and usability ground to just use a shorter passcode timeout (like 30 minutes).
Or use a 1% chance that it will prompt for password after fingerprint... or 100% chance when you use the wrong finger. But how to tell? When the password prompt pops up, it might have been the 1% random chance.
If you set it to 30 minutes, you'll be typing in the PIN/password practically every time you grab it.
